hwmon: (mlxreg-fan) Fix double "Mellanox"

Remove the duplicate "Mellanox" in the help text for the Mellanox FAN
driver configuration option.

Fixes: 65afb4c8 ("hwmon: (mlxreg-fan) Add support for Mellanox FAN driver")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20201005124843.26688-1-geert+renesas@glider.be


Signed-off-by: Guenter Roeck <linux@roeck-us.net>